Such devices are well known per se. Typically a strap attached to the object is passed around the post and tightened and the ends secured together by means of a buckle, or by a screw adjustably connecting the strap ends.
A well known form of buckle for this purpose comprises a plate having a strip cut and pressed out of its central region providing a bridge around which one end of the strap can pass, and leaving a slot through which the other end of the strap passes, the end of the strap taken around the bridge being then retained by pressing down onto it a pair of ears upstanding from the plate at one end. Examples of such buckles are sold under the Trade Marks "Tespa" and "Band-it".
One problem associated with fixing signs to circular posts is that the sign can sometimes be shifted angularly around the post, e.g. by the action of wind or other pressure. To combat this it has been proposed to provide the strap with inwardly projecting depressions in the strap material which, in the case of a plastics coated steel pole, press into the plastics coating and resist displacement around the pole.